This invention relates to an aluminum electrolytic capacitor capable of operation at 200 VDC or higher at an ambient temperature of 130.degree. C. through the use of an electrolyte containing mono(di-n-propylammonium) adipate or mono(diisopropylammonium) adipate as solute, and a phosphate salt and water dissolved in ethylene glycol as solvent.
Heretofore, electrolytes for aluminum electrolytic capacitors operating at 200 V or higher most commonly contained salts of boric acid or boric acid derivatives as the solute in ethylene glycol as solvent. The maximum operating temperature for such an electrolyte system is less than 100.degree. C. and normally 65.degree.-85.degree. C. The temperature limitation is due to the facile reaction of glycol with boric acid and other borate species to form polymeric glycol-borates and water. The minimum operating temperatures are above -20.degree. C. inasmuch as glycol freezes at -17.4.degree. C.
The effective temperature operating range of aluminum electrolytic capacitors has been expanded in both directions by replacing the glycol solvent with N,N-dimethylformamide (DMF) which has a boiling point of 153.degree. C. and a freezing point of -61.degree. C. There are known prior art DMF electrolytes that can be effectively used over the temperature range -55.degree. C. to 125.degree. C. However, DMF is a very aggressive solvent and attacks most materials of construction. The most resistant material for sealing gaskets and O-rings is Butyl rubber. Unfortunately, DMF will be transmitted through a Butyl rubber closure at a rate that increases with increasing temperature, thus limiting the life of the capacitor since the capacitors will not function adequately when approximately one-half the solvent has been lost.
DMF also has a flash point of 67.degree. C. making it undesirable for use as solvent in capacitors that are to be used in confined spaces. In contrast, glycol has a boiling point of 197.2.degree. C. and a flash point of 116.degree. C. and is much easier to contain. Rates of transmission of glycol through both Butyl rubber and ethylene-propylene rubber (EPR) are almost negligible.
For current power supply operations, it is desirable to provide an aluminum electrolytic capacitor capable of operating continuously at 200 VDC or higher at an ambient temperature of 130.degree. C. with modest low temperature properties.
It would be desirable to use ethylene glycol as solvent for the reasons given above. If glycol is used, then the solute can not be boric acid or a borate because of its reaction with glycol as described above. The solute should be one that will not react with glycol or any other cosolvent that might be used. The solute must also be stable at operating temperatures of 130.degree. C., and at somewhat higher temperatures.
The major cause of resistivity increase in an electrolyte is amide formation, particularly where the solute is an ammonium or substituted ammonium salt of a dicarboxylic acid. For example, diammonium adipate, a known solute for electrolytic capacitor electrolytes, will rapidly form adipamide, a non-conducting species, at 125.degree. C. when used in an ethylene glycol solvent. Since adipamide is insoluble in glycol, this reaction is readily detected. For other salts that undergo amide formation but form soluble amides, the reaction can be detected by increases in resistivity. Amide formation proceeds most readily with ammonium salts, and more readily with salts of primary amines than with salts of secondary amines. Amide formation is most difficult with salts of tertiary amines, as a carbon-nitrogen bond must be broken for it to proceed.